The present invention relates to a toner for developing electrostatic latent images in electrophotography, electrostatic recording and electrostatic printing.
A toner used in electrophotography is added with a fluidizing agent such as silica, titania and alumina in order to improve toner fluidization, toner cleaning properties and the like. Silica is generally added because of easiest availability. When only silica, however, is added to a toner, there arise problems such as high charging level at initial stage, environmental instability and deterioration of fluidizing properties caused by toner burying. In order to prevent such problems, a fluidizing agent such as titania or alumina is further added in addition to silica.
When fluidizing agents are added in combination as above mentioned, high charge amount at initial stage caused by silica is restrained and toner can be charged speedily to an adequate charging level. Therefore, toner can be made excellent in image properties at initial stage. Titania or alumina, however, has essentially lower charging level than silica. When used for a long time, toner is influenced adversely by alumina or titania to bring about problems such as lack of charge amount and toner scattering, further accompanied by toner fogs. Further, as toner is consumed, the essential problem that toner comes to lack charge amount after used for a long time in spite of toner supply can not be solved.
On the other hand, multi-color copy images can be formed by laminating various color toners. Such a color toner is generally composed of resin of lower softening point than that of resin used for a conventional black toner. It is necessary to treat the surface of toner with a large amount of inorganic particles in order to achieve fine texture of solid copy images. Silica itself has high electrical resistance and large specific surface area. When only silica is added to a toner as inorganic fine particles, the silica gives the toner so high charging ability that density of copy images becomes low. In particular, this problem is remarkable under low humid conditions. When only titania is used as inorganic fine particles, there arise problems such as lack of toner charge amount, fogs on copy ground and toner scattering because titania has relatively large particle size and low electrical resistance. These problems are remarkable under high humid conditions.